Antenna Introduction

There are a ton of projects online that you can find for LF band antennas. These range from various loop antennas, random wire, inverted V style antennas, and countless other designs. You can even find some kite antennas! A great antenna has a resonance for the desired operating frequency, good SWR, and some gain to focus the energy to places we want it to go. Unfortunately, given the Part 15 restrictions and absurd wavelength of about 1 mile, we really can't do any of this. Most LowFERs make up for this deficiency on the receive side. The receive antenna can be unlimited in size, which is usually combined with a good receiver.

After my last disaster of trying to make a "simple" Yagi antenna for 70 cm satellite work (my son uses the ruins of this poor Yagi project as swords now), I decided to see what could be done with a pair of wire strippers and some wire. Even a well built poster boy antenna will do poorly on the LF band, but lets see what antennas a loafing LowFER an accomplish without a trip to the hardware store, no parts, and no antenna matching. Or..... lets just pretend you are being held prisoner behind enemy lines, and they made the unfortunate decision of leaving an Arduino and some scrap wire in the cell....

"1 Minute" Untuned Antenna Circuits

The Jumper Antenna

The Jumper Antenna was just one of those male/male jumper wires that are useful for breadboards. It wasn't very long, perhaps 4". It was connected to nothing on the other end.

Results: Worked great on the kitchen table, but not much further than that.

The Wire Antenna

The FCC lets you use up to 50 feet of wire for an antenna. Lets stick the wire into one of the header pins and see what happens. To avoid a trip hazard, I just laid it on the floor. It was connected to nothing on the other end.

Results: Not bad, but not all that great. It covered the house but not much more. The signal was considerably weak but better than the jumper wire I used.

The Wire Antenna in a Loop

Everyone seems to be on to something good with these loop antenna designs. Using the dining room chandelier as a support, I was able to construct a very crude loop antenna. The final end of the wire was connected to an unconnected pin on the same breadboard.

Results: It actually improved over the long wire on the floor. I was able to receive it outside the front door. I suspect that there might be some weak coupling with ground taking place, making the radio circuit slightly more efficient.

A Wire Loop Connected to Ground

A word of warning: do not connect an untuned, unmatched antenna circuit directly to ground. This will damage your Arduino.

Connecting a PWM pin to ground will force a relatively large amount of current to flow through the wire. Some of this energy will be radiated out. In order to avoid damage, I was able to rig up a 194 ohm resistor (100 + 47 + 47), which should limit the current to about 25 mA. We can get over 100mW flowing through this antenna.

Results: If my Sony is anywhere near the loop, it overloads the receiver AGC. Wow. That was a big improvement. I can get about 100 feet now. If you just adhere to the Absolute Maximum Range (40mA), it does boost a bit -- but you risk burning out that pin.

Wire Loop with 100 pF Decoupling Capacitor Connected to Ground

Maybe blocking the DC component from generating heat into the resistor(s) will help? I will use a de-coupling capacitor (the 100pF I found in a drawer) and connect the other end of the loop to ground.

Results: This significantly dropped the radiated power level worse than my basic wire antenna. If I disconnect the ground and touch the conductor, it does give the signal a slight boost...interesting. Besides our obvious antenna matching problem, a better ground system is going to help us improve later.

Conclusion

While some of these simple wire designs partially worked, we are going to need to do a little work to design a tuned antenna circuit.

In the next update, we will take a look at the resonance circuit from an AM transmitter project by Markus Gritsch and modify his simple LC resonator circuit for use in our LowFER free band.

We will also take a look at our raw PWM signal on a spectrum analyzer and see if we can build a Low Pass Filter to knock out some of these nasty harmonics.

Here is a python script we can use to calculate the values. Note that 0.015 uF and 47 uH are some pretty standard capacitor and inductor values for that LC resonator circuit:

from __future__ import division
import math
from math import log

pi = 3.1416
l = 0.000051    # 47uH is .000047  
c = 0.000000015 # 1 nanofarad = .000000001


fres = 1 / (2 * pi * math.sqrt(l * c)) 

print fres